1. Field of the Invention
The present invention concerns a circular microwave window that can work at high power in a wide frequency band.
Microwave devices that work at different atmospheric pressures require the use of microwave windows. The windows are designed to insulate these devices from atmospheric pressure, but they enable the propagation of microwaves without introducing reflection or internal resonance.
Microwave tubes generally work at an extremely low pressure while certain components such as circulators, isolators, coaxial lines and waveguides may contain a gas taken to a pressure that is higher than atmospheric pressure to improve their behavior under power.
Consequently, a microwave window should have solidity sufficient to withstand pressures of the order of several kilograms per square centimeter, without damage.
Moreover, a microwave window should withstand short-lived anomalies such as electric flashes, mechanical stresses such as shocks and vibrations, and temperature variations which may be great, especially if the window is brazed to a frame or into a waveguide. If this is not the case, there is the risk that the window might break, and then air will enter the microwave device and cause it to deteriorate.
Moreover, it is desirable for microwave windows to be capable of being used in a wide frequency band. This frequency band will correspond to the working frequency band of the microwave devices in which they are mounted.
In this frequency band, the windows should not have stray internal resonance modes, also known as ghost modes.
Furthermore, in this frequency band, the standing wave ratio should be low and, consequently, there should be a small degree of reflections.
2. Description of the Prior Art
Among prior art microwave windows there is, notably, the pillbox window. As shown in FIGS. 1a, 1b, this window is formed by a thin strip 1 of a dielectric material brazed into a circular waveguide section 2. The circular waveguide section 2 is connected on either side to a rectangular waveguide 3.
As is shown more particularly in FIG. 1b, the diameter D of the circular guide 2 (which is also that of the dielectric strip 1) is substantially equal to the diagonal of the rectangular guide 3.
These dimensions make it possible to keep substantially the same wavelengths of guided electrical waves in the rectangular guide 3 and the circular guide section 2.
Furthermore, the length L of the circular waveguide section 2 is chosen so as to be substantially equal to half of the guided wavelength 1.sub.g.
The pill-box type window behaves like a half-wave matching transformer. The result thereof is that the matching is accurate at the working center frequency but gradually deteriorates on either side.
This type of window has many ghost modes, and this fact reduces its effective working bandwidth to about 12% as compared with the center frequency.
Furthermore, owing to its dimensions, the pill-box window is relatively brittle but, technologically, it is easy to make.
There are sturdy microwave windows in existence. These are generally made of thick ceramic. They are mounted in a rectangular waveguide and block the entire cross-section of this guide. They have the shape of a rectangular parallelepiped and their thickness corresponds to a half wavelength of the working center frequency in the rectangular waveguide for a window made of alumina.
These windows have a working band that is extremely restricted because of the thickness of the ceramic. The working bandwidth is about 5% to 7% of the working center frequency in the rectangular waveguide for a window made of alumina.
Another type of window has been described in the French patent No. FR 2 558 306 filed on 17th January 1984. This window is derived from the pill-box window but has a widened working band.
As is shown in FIGS. 2a, 2b, the window has a thin circular strip 6 made of dielectric material brazed into a circular guide section 7. The diameter D' of the strip is equal to the diameter of the circular guide 7. The ends of this circular guide section 7 are connected to a rectangular guide 5.
The window is equivalent to a resonant volume within which undesirable ghost modes may develop. Within the circular waveguide section 7, and at the frequencies used, it is the TM.sub.010 and TE.sub.111 modes that may appear. If these modes are not to be excited, the diameter D' of the strip 6, its thickness e' and the length L' of the circular guide section 7 have to be carefully chosen. Experience shows that it becomes necessary to reduce these three dimensions as compared with those of standard pill-box windows.
Thus, the diameter D' is chosen to be between the sizes of the large side and the small side of the rectangular waveguide 5. Consequently, self-inductance shutters 8 are created at the rectangular guide in order to obtain a matching of the window with the center frequency of the working band of the rectangular waveguide 5.
The thickness e' of the strip 6 is chosen to be as small as possible, but sufficient to withstand the mechanical and electrical stresses to which it is subjected.
A half-wave matching transformer 10 is added. This transformer 10 is formed by two elements of the same length, placed on either side of the circular waveguide section 7, within the rectangular waveguide 5, so that they overlap at least one of its large sides. This transformer enables sufficient matching to be achieved throughout the working frequency band of the rectangular waveguide 5.
With this window, we obtain an effective bandwidth corresponding to about 40% of the center frequency.
However, this window is more complicated to make than the pill-box window. The diameter of the circular waveguide 7 is between the sizes of the large side and the small side of the rectangular waveguide 5. Consequently, additional pieces have to be used to provide for impervious sealing between the interior and the exterior of the guides, these pieces being placed at the junction between the rectangular guide 5 and the two ends of the circular guide section 7. Besides, the fact that the length of the circular guide is small in relation to its diameter does not favor its flexibility which is necessary to allow for differences, in expansion, between the ceramic and the guide.
The window according to the invention is more solid than the standard pill-box window. It is also easier to make and works in a widened frequency band without having ghost modes.
Its bandwidth performance characteristics are lose to those of the window described in the French patent No. 2 558 306, but it will be easier to make. Since its dimensions are greater than those of the window described in the patent No. FR 2 558 306, it can work at a higher peak power value. Furthermore, since the length of the circular guide has been increased with respect to that of the circular guide of the window of the patent No. FR 2 558 306, the flexibility of the guide is improved.